1. Field of the Invention
The present invention relates to a heat transfer pipe. More particularly, it relates to a head transfer pipe wherein a medium to be cooled is passed along its outer surface and a cooling medium is passed along its inner surface, both surfaces of which, respectively, have a specified configuration thereby maximizing the efficiency of heat exchange between these media by means of these configurations.
2. Description of the Prior Art
A construction for a refrigerant condenser used in refrigerators, coolers and the like has been generally known to have a plurality of heat transfer pipes which are disposed within a cylindrical shell and a medium, e.g. refrigerant gas, flowing on the outer surface of the pipes being condensed by a cooling medium such as water flowing within the pipe. For an improvement of the efficiency of heat exchange of such heat transfer pipes, many approaches, such as the following, have been studied;
(i) An increase in the effective area of heat exchange per unit length of the heat transfer pipe by forming spiral fins on the outer surface of the pipe by means of a rolling tool, thereby manufacturing a so-called "low fin heat transfer pipe" 100 as shown in FIGS. 1 and 2. The condensate of the refrigerant which forms during heat exchange would remain on the surface of the fins 111, the shape of which is rectangular in cross-section, to form liquid film f which substantially decrease the efficiency of heat transfer. On the other hand, it may be possible to increase the number of rows of fins in the axial direction of the pipe so as to substantially enlarge its outer surface. However, the narrow grooves between the fins are filled with the condensate during heat exchange so that an increase in the efficiency of heat transfer cannot be expected. It is also possible to form taller fins, which require large shells to dispose therein these fins, resulting in large and heavy condensers. In the case where the condensers are assembled by a shell-and-tube system, its assembly is troublesome and labor consuming.
(ii) A decrease in the formation of liquid film on the surface of the fins by plowing or cutting the fins to make their tips thin as shown in FIG. 3. The thin tips of the fins allow a certain amount of condensate to flow immediately into the grooves 120 between the fins 111 so that the formation of liquid films f is decreased, thereby improving the efficiency of heat transfer. However, since the formation of liquid film is unavoidable the resulting efficiency of heat transfer is still lower;
(iii) Prevention of the formation of liquid film by treating the heat transfer surface of the pipe by means of a coating. This approach is theoretically advantageous but impractical because such a surface treatment technique is still incomplete at this moment;
(iv) An improvement in the efficiency of heat transfer on the inner side of the pipe. This is not effective when the shell side heat transfer coefficient is more dominant than the tube side heat transfer coefficient because it is not intended to eliminate the condensate of the refrigerant on the outer surface of the pipe.
All the above-mentioned approaches cannot provide heat transfer pipes having a high efficiency of heat transfer while being small in size and light weight in construction.
U.S. Pat. No. 4,330,036 discloses a heat transfer pipe, on the outer surface of the body of which a number of fins are integrally formed. Although, the fins are divided into a plurality of sections by spirally running breaks with a predetermined pitch to enlarge the effective area of heat exchange of the pipe, the resulting efficiency of heat exchange, especially the efficiency of condensation heat exchange with relation to the outer surface area of the pipe, is still insufficient.